14 AUSTRALIAN SKY & TELESCOPE February | March 2019
Allthreeprojectsstartedcollectingpulsartimingdataat
leastadecadeago,andallhaverelativelysimilarcapabilities
and sensitivities. An array’s frequency range depends on how
long it’s been operational; currently, the arrays span nanohertz
to millihertz wavelengths, with a sweet spot between 3 and
10nHz,saysAlbertoSesana(UniversityofBirmingham,UK).
NANOGrav and EPTA observe many of the same Northern
Hemisphere pulsars, whereas PPTA concentrates on those visible
from the Southern Hemisphere. Combined, they currently
watch roughly 75 pulsars, adding several new ones each year.
NANOGrav, EPTA and PPTA are on the lookout for two
different kinds of sources. They can all catch the rumbles of
individualblackholebinarieswithinseveralhundredmillion
light-years. But NANOGrav team member Scott Ransom
(National Radio Astronomy Observatory) says individualTHOWITWORKSGravitational waves ripple out from an inspiralling pair of supermassive black holes, slightly stretching and squeezing the
spatial dimensions that are perpendicular to the waves’ direction of motion (A, in 3D then with 2D cross sections). When these waves pass Earth
andnearbypulsars,theychangethedistancebetweeneachpulsarandEarth(B,asseenlookingdown on the crests in A). The white arrows
in the main graphic indicate how much the distance changes for each pulsar, determined by the angle with respect to the wave’s direction of
motion. As a pulsar’s distance oscillates, the arrival times of its signals change (see facing page). Because the pulsars lie at different distances
from both Earth and the waves’ source (white lines), different parts of the wave hit each pulsar at any given time. This difference means that each
pulsar’s timing shift probes a distinct slice of the gravitational wave pattern (C). By combining the changes in arrival times for many pulsars in
differentpartsofthesky,astronomersshouldbeabletodeterminewherethegravitationalwav om and what created them.sources probably won’t be their first detection.
Instead, it’ll be the combined gravitational-wave signal
of all the inspiralling supermassive black hole binaries
over time, called thestochastic background. The stochastic
background is like a cacophony of voices in a football
stadium, where it’s impossible to distinguish any single
conversation. The contributing binaries will typically have
black holes containing 100 million to 10 billion solar masses,
with separations of just a few thousandths of a light-year, and
orbital periods measured in years to decades.
Teasing out this background signal is an exceedingly
difficult task, because it consists of the superposition of
gravitational waves of different strengths and wavelengths
coursing through our corner of the galaxy from all directions.
The signal looks very different than waves from a specificLEAH TISCIONE /S&TGravitational
waveDirection of
Direction of propagation propagation Pulsars probe different parts of waveABCTHE NEXT GRAVITATIONAL-WAVE REVOLUTION